1. Field of the Invention
The present invention relates to a wireless communication system, wireless communication device and method, and computer program for mutual communication among a plurality of wireless stations, and particularly to a wireless communication system, wireless communication device and method, and computer program in which a network is constructed under the management of a specific control station.
More specifically, the present invention relates to a wireless communication system, wireless communication device and method, and computer program for wireless communication having both a region for asynchronous information transmission at arbitrary timing and a region for information transmission with channel time allocation, and particularly to a wireless communication system, wireless communication device and method, and computer program in which information is transmitted initially via asynchronous communication and then, if the information is not successfully transmitted via asynchronous communication, via channel time allocation communication.
2. Description of the Related Art
A plurality of computers are connected to construct a LAN (local area network) so that files and data can be shared or peripheral devices such as a printer can be shared and information can be exchanged by transferring e-mail or data content.
In the past, devices were typically connected to a LAN using lines such as optical fibers, coaxial cables, and twisted pair cables. Such LAN connections require cables to be installed, thus making it difficult to construct a network with ease and increasing the complexity of cable connections. After construction of a LAN, another difficulty occurs in that device mobility is restricted by the cable length. Accordingly, wireless LAN systems have become increasingly popular as a solution to the wiring problem with such traditional wired LAN systems. This type of wireless LAN system can remove most of the cables in working environments such as offices, thus allowing users to relatively easily move communication terminals such as personal computers (PCs).
With the advent of high-speed and low-cost wireless LAN systems in recent years, the demands for such wireless LAN systems have increased remarkably. Recently, in particular, studies on personal area network (PAN) systems have been made to construct a small-scale wireless network among a plurality of electronic devices around individuals for information communication. A variety of wireless communication systems using unlicensed frequency bands such as the 2.4-GHz band and the 5-GHz band are standardized.
For example, the IEEE 802.15.3 working group is standardizing the specification of high-speed (20 Mbps or greater) wireless personal area networks. In IEEE 802.15.3, standardization mainly focusing on a PHY layer using 2.4 GHz-band signals is being developed.
In this type of wireless personal network, a single wireless communication device operates as a control station called a “coordinator”, and a personal area network with a range as short as approximately 10 m from this coordinator is formed. The coordinator sends beacon signals at predetermined intervals, and the intervals of the beacon signals are defined as transmission frame periods. For every transmission frame period, channel time is allocated to or reserved for each wireless communication device.
In one of the related art wireless communication methods, specific parameter information is exchanged prior to transmission, and then channel time used for the transmission is allocated or reserved, after which the communication starts. A well-known channel time allocation communication method is the BRAN (Broadband Radio Access Network) HiperLAN Type-2 wireless communication system, which is an European wireless local area network (LAN) standard.
Another related art wireless communication method is a method for information transmission at arbitrary timing without channel time allocation. A famous information transmission method without channel time allocation is the access control protocol for use in the wireless communication system of IEEE 802.11 wireless local area networks (LANs).
For example, when AV streaming is implemented as an upper-layer application, the wireless transmission channel is occupied for a long time and it is therefore effective to use the former wireless communication method, which enables the bandwidth to be reserved for a long time.
On the other hand, when asynchronous information exchange such as IP-packet-based transmission is implemented as an upper-layer application, it is effective to use the latter wireless communication method, which provides immediate transmission when the demand for transmission occurs.
A wireless communication system using both communication methods is represented by, for example, the high-speed wireless personal area network (PAN) communication protocol defined by the IEEE 802.15.3 standard. A MAC (medium access control) layer specified by IEEE 802.15.3 includes a contention access period (CAP) and a contention free period (CFP). During the CAP, asynchronous communication is performed to exchange short data or command information. For stream communication, on the other hand, dynamic time slot allocation is performed using a guaranteed time slot (GTS) in the CFP, thereby achieving channel time allocation communication.
In the IEEE 802.15.3 standard, the MAC layer is defined so as to be applicable as a standard specification of a PHY (physical) layer other than the PHY layer using 2.4 GHz band signals. According to the IEEE 802.15.3 standard, a PHY layer other than the PHY layer using the 2.4 GHz band signals is to be utilized.
Japanese Patent Application No. 2002-28128 assigned to the present assignee discloses a wireless communication method in which a part of the transmission bandwidth is used for channel time allocation communication while information transmission at arbitrary timing is performed in the remaining bandwidth. Specifically, a wireless communication device sets a management information announcement period at predetermined time intervals, and sends management information including reception timing information indicating its information reception start position, reception window information, and reception period information. A recipient wireless communication device which receives the management information stores the reception timing information, the reception window information, and the reception period information in association with the communication device number of the originating wireless communication device. For information transmission, based on the reception timing, reception window, and reception period information of a communication partner, the reception start position of the corresponding communication device is obtained to transmit information at this given timing. According to this wireless communication method, a wireless communication device announces its reception timing, reception window, and reception period information beforehand, and receives information in this given window. This eliminates the need for this wireless communication device to constantly wait, unlike the traditional method, thus reducing the power consumption for reception.
Japanese Patent Application No. 2002-57839 assigned to the present assignee discloses a wireless communication method in which information transmission is performed not at arbitrary timing, but at a reception timing predetermined for a specific communication device. Specifically, a control station specifies a given frame and assigns a unique access slot in this frame, for a period in which data is received, to each wireless communication device in this network to send a beacon signal including the assigned access slot. Each wireless communication device in the wireless network receives information during its unique access slot defined by the beacon signal. For transmission of information between communication devices in the network, a given communication device transmits the information using the access slot of the destination communication device. According to this wireless communication method, each wireless communication device receives information in the access slot assigned thereto based on the beacon signal. This method enables the information to be received in a simple manner, and also eliminates the need for the wireless communication devices in the network to constantly wait, thus reducing the power consumption of the devices. Furthermore, each wireless communication device which receives a beacon signal from the control station can easily determine the reception timing of the other wireless communication devices. Therefore, communication devices can synchronize access slot timing based on the beacon signals, thereby accomplishing timing synchronization within the network, thus making the reception process simple. For data transmission, a given wireless communication device transmits information to a destination device using the access slot of the destination device, thus achieving highly random-access asynchronous communication.
A communication sequence of the related art using channel time allocation, which includes exchanging channel time allocation parameters necessary for transmission according to an instruction from an upper layer such as an application layer and making a request according to the parameters, is employed as a channel time reservation method for isochronous transmission in wireless 1394 (ARIB-STD-T72).
In a wireless communication scheme using channel time allocation, channel time must be allocated according to a predetermined procedure before information transmission. One problem is that information cannot be transmitted until completion of channel time allocation.
In a wireless communication scheme for information transmission at arbitrary timing, all the communication devices forming a network must constantly wait because they cannot determine which communication device sends information at which timing.
In principle, a wireless communication protocol defining both the above-mentioned wireless communication schemes cannot determine which wireless communication scheme is to be used for information transmission unless the type of upper-layer application is identified.
If it cannot be determined immediately after a transmission demand whether or not this transmission was requested by an application that requires channel time allocation communication, information must be redundantly buffered and it must be determined, based on the amount of buffered information, whether or not the transmission was requested by an application that requires channel time allocation. Therefore, immediate wireless transmission is not achievable.
In the channel time allocation communication protocol, extra channel time allocation for a small amount of information transmission causes a great portion of traffic on the wireless transmission channel to be occupied by communication required for the channel time allocation, with the result that the throughput of the information transmission is reduced.
Furthermore, channel time allocation for a small amount of information which must be returned in a short time, such as response information at upper layers, can cause a delay in processing.
In a channel time allocation transmission sequence of the related art, channel time allocation parameters are specified according to an instruction from an upper layer such as an application layer to request channel time allocation according to the parameters. In such a sequence, a mechanism for exchanging the parameters beforehand must be defined.
Therefore, wireless communication cannot be closed and controlled using the wireless communication protocol alone.